Shedding Light on the Compton-thick Active Galactic Nucleus in the Ultraluminous Infrared Galaxy UGC 5101 with Broadband X-Ray Spectroscopy
- Department of Astronomy, Kyoto University, Kyoto 606-8502 (Japan)
- National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo 181-8588 (Japan)
- Department of Physics, Ehime University, Bunkyo-cho, Matsuyama, Ehime 790-8577 (Japan)
- Instituto de Astrofisica, Pontificia Universidad Catolica de Chile, Casilla 306, Santiago 22 (Chile)
We report the broadband X-ray spectra of the ultraluminous infrared galaxy (ULIRG) UGC 5101 in the 0.25–100 keV band observed with the Swift/Burst Alert Telescope (BAT), Nuclear Spectroscopic Telescope Array (NuSTAR), Suzaku, XMM-Newton, and Chandra. A Compton-thick active galactic nucleus (AGN) obscured with a hydrogen column density of ≈1.3×10{sup 24} cm{sup −2} is detected above 10 keV. A spectral fit with a numerical torus model favors a large half-opening angle of the torus, >41°, suggesting that the covering fraction of material heavily obscuring the X-ray source is moderate. The intrinsic 2–10 keV luminosity is determined to be ≈1.4×10{sup 43} erg s{sup −1}, which is ≈2.5 times larger than the previous estimate using only data below 10 keV with a simple spectral model. We find that UGC 5101 shows the ratio between the [O iv] 26 μm line and 2–10 keV luminosities similar to those of normal Seyfert galaxies, along with other ULIRGs observed with NuSTAR, indicating that a significant portion of local ULIRGs are not really “X-ray faint” with respect to the flux of forbidden lines originating from the narrow-line region. We propose a possible scenario that (1) the AGN in UGC 5101 is surrounded not only by Compton-thick matter located close to the equatorial plane but also by Compton-thin (N{sub H}∼10{sup 21} cm{sup −2}) matter in the torus-hole region and (2) it is accreting at a high Eddington rate with a steep UV to X-ray spectral energy distribution. Nevertheless, we argue that AGNs in many ULIRGs do not look extraordinary (i.e., extremely X-ray faint), as suggested by recent works, compared with normal Seyferts.
- OSTI ID:
- 22869490
- Journal Information:
- Astrophysical Journal, Vol. 835, Issue 2; Other Information: Country of input: International Atomic Energy Agency (IAEA); ISSN 0004-637X
- Country of Publication:
- United States
- Language:
- English
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